Method and apparatus for clarifying and/or thickening a slurry

ABSTRACT

The invention relates to a method of clarifying and/or thickening a solution containing solids i.e. a slurry, and the apparatus for accomplishing this. Using the method and apparatus of the present invention, the slurry to be fed into the clarification or thickening apparatus is distributed evenly into the slurry already in the tank. It is characteristic of the clarification tank now developed that the displacement capacity of the raking system in the tank is the same from the edges of the tank up to the centre. The displacement capacity is measured as being the same as the underflow discharge capacity from the centre of the tank. The displacement capacity can be determined to be the same using rake displacement plates of different heights, according to the distance of the displacement plates from the rake shaft and the number of plates rotating on the same ring-like cross-section.

This invention relates to a method of clarifying and/or thickening asolution containing solids i.e. a slurry, and the apparatus foraccomplishing this. Using the method and apparatus of the presentinvention, the slurry to be fed into the clarification or thickeningapparatus is distributed evenly into the slurry already in the tank. Itis characteristic of the clarification tank now developed that thedisplacement capacity of the raking system in the tank is the same fromthe edges of the tank up to the centre. The displacement capacity ismeasured as being the same as the underflow discharge capacity from thecentre of the tank. The displacement capacity can be specified to be thesame using rake displacement plates of different heights, according tothe distance of the displacement plates from the rake shaft and thenumber of plates rotating on the same ring-like cross-section.

The clarifying and thickening apparatus comprises a tank, into which theslurry to be clarified is fed via a feedwell, which is located in thecentre of the clarification tank, generally in its upper section. Whenthe term clarification is used later in the specification of theinvention, it is used however to refer also to thickening, since themethods and apparatus described can usually be applied for bothpurposes. The diameter of the clarification tank is ten times largerthan that of the feedwell. In addition to the slurry a flocculant usedin clarification is fed into the feedwell. The flocculant is often mixedinto the slurry while it is being fed into the feedwell. Theclarification or thickening tank may be equipped with a rake operatingnear the bottom, or function without one.

The clarified liquid, or overflow, is removed from the clarificationtank as an overflow and the thickened slurry, or underflow, is removedfrom the central part of the tank bottom, to where it has been moved bythe rake arms. The bottom of a thickening tank is usually inclined, i.e.sloping down towards the centre, whereby the removal of solids is madeeasier. The number of arms in the rake can be varied depending on thedesign solution. Some of the rake arms may be the length of the tankradius, others may be shorter. Vertical plates are fixed to the rakearms, and they move the slurry from the edges towards the centre. Theyare fixed either perpendicularly to the rake arm or for example at anangle of 30 degrees to it.

When the mixing of the flocculant into the slurry occurs in the feedwellwith only for instance the mixing achieved by a tangential feed, it isclear that the flocculant does not mix evenly and that the local shearforces become large. However, the flocculants used in the present dayare large-molecule polymers whose action is hindered as shear forcesincrease. If the slurry to be transferred to the clarification tank fromthe feedwell is discharged there forcefully, there is a danger that itwill be aimed basically upwards in the direction of the smallestpressure and thus be mixed into the separated, already clarifiedoverflow in the upper section of the tank. The clarification orthickening tank rakes usually function so that they have an “overlarge”displacement capacity, whereby they press the thickened slurry to thecentre of the tank, where a pile rises that is higher than the rest ofthe slurry.

A method and apparatus has now been developed to clarify and thicken asolution containing solids, in other words a slurry, where the capacityof the rake located in the clarification tank is dimensioned to be inprinciple the same magnitude from the edges of the tank to the centralarea. The slurry is fed into the actual thickener via a feedwell so thatthe slurry spreads outwards and downwards from the well. The thickener,which acts as a settling space, is equipped with a rake in the directionof the bottom, of which at least two of the displacement elements reachthe edge of the cylindrical tank and at least two are clearly shorter.The invention is not however limited to this displacement element orrake arms solution. The height of the rake arm displacement plates canbe varied so that the displacement capacity of each circle formed by therake arm displacement plates is more or less the same up to the centralarea. In order to achieve this the height of the displacement plates inthe apparatus according to the invention is dependent on the distance ofthe plate from the rake shaft and on the number of rake arms at thepoint in question. The displacement plates are preferably inclinedvertically towards the rake shaft. The essential features of theinvention will be made apparent in the attached claims.

The method of the present invention is especially applicable to caseswhere the aim is a clear overflow solution, completely separated fromsolids. The settled and removed underflow can be recirculated back toearlier process steps. Only the equivalent amount of accumulatedunderflow is taken away for instance via filtration. In this way thehydrodynamic behaviour of the settling space is increased. The methodand apparatus have proven beneficial particularly in the treatment ofsediment containing gypsum and metal hydroxide. The sediment is formedwhen the acidic and rinsing waters generated in steel pickling areneutralized with lime compounds.

According to the invention there is a feedwell inside the clarificationor thickening apparatus, into which the solids-containing solution orslurry is fed. The slurry can be fed into the feedwell for instance fromabove, but nevertheless so that the slurry feed is gentle. Theflocculant used is also fed into the feedwell. The feedwell ispreferably an upright cylinder equipped with its own mixing element. Themixing element is preferably a helix-type mixer working on the principledescribed in U.S. Pat. No. 5,182,087, with a structure of two tubescircling around a shaft, making ⅓-2 revolutions around the shaft. Themixing direction is rising. Thanks to the mixing element it is possibleto mix the flocculant into the slurry evenly and the mixing is of verylow and uniform intensity throughout the feedwell zone. Thus stronglocalized mixing can be avoided.

The diameter of the mixing element is 40-80% of that of the feedwell.The mixing element shaft is a hollow cylinder, with a diameter largeenough so that the shaft of the clarification tank rake and also thecoupling flange of the shaft fit through the mixing element shaft. Thesupports required by the helix tube are fixed at one end naturally tothe tubes and at the other to the shaft casing of the mixing element.The feedwell can also be equipped with baffles, which are placed atequal intervals along the edge of the well. Some of the baffles are theheight of the feedwell but preferably interspersed with bafflesextending from the bottom upwards to a height of only about ⅓ of thelong baffles and the total height of the feedwell.

The slurry and the flocculant mixed into it are guided from the actualfeedwell downward via openings situated at the lower edge of thefeedwell, and always located between the baffles. The openings are infront of the baffles in relation to the rotation direction of the mixer.Thus the pressure pulses caused by the mixer at the point of bafflespromote the even discharge of the slurry at each opening. The velocityof the outflow is preferably of the order of 0.05-0.2 m/s. Some of theslurry may also flow through the rake shaft socket.

From the feedwell the slurry flows to a guiding cone located below thefeedwell, which extends into the upper part of the slurry layer in theclarification tank. The purpose of the guiding cone is to give acomponent of downward movement to the slurry entering the settlingspace, reducing the tendency of the slurry to curve upwards. In this waythe slurry spreads evenly into the compacted sediment without mixing theoverflow solution. The guiding cone, as its name suggests, is adownwardly widening cone, which is open on its inner side. The slurryfrom the feedwell is discharged through the openings in the lower partof the edges of the guiding cone into the clarification tank. The solidlower edge of the guide pushes down the compacted slurry, and thepressure differences in the tank cause the slurry to divide itselfevenly among the various discharge ports. The guiding cone is situatedin the clarification tank in the upper part of the slurry layer, whichis still in the compaction stage. It has been shown that the preferableheight of the lower section of the guiding cone is 0.5-0.7 times thedepth of the solution and slurry in the centre of the tank.

In the settling space, in other words the clarification tank, theunderflow, which has settled downwards and compacted at the same time,is moved from the edges of the cylindrical tank towards the centre ofthe tank using a rake system. If the tank is not cylindrical, the movingof the underflow that has settled in the corners of the tank to acircular boundary in the tank must be performed in a way known before.In the apparatus now developed the rake has two arms reaching the edgesof the cylindrical tank and between them two arms that are only abouthalf the length of the long arms. It is of course clear that the numberof long arms and auxiliary arms can be varied within the framework ofthe invention without being limited to two long and two auxiliary arms.It is characteristic of the clarification tank solution now developedthat the underflow displacement capacity is the same from the edges ofthe tank to the central area. The displacement capacity is measured asbeing the same as the underflow discharge capacity from the centre ofthe tank. The method enables the avoidance of overlarge displacementcapacity, which result in a growing amount of compacted slurry in thecentre of the clarifier, which rises up in piles. The surface of thecompacted underflow is not allowed to rise as far as the feedwellguiding cone, and thus an uneven and channelised feed of slurry isavoided.

A uniform displacement capacity in the settling space is achieved sothat the height of the underflow displacement plates attached to therake arms varies. The displacement plates that are at the same distancefrom the rake shaft move the underflow from the ring-like area inwards.Since the cross-section of the ring-like area gets smaller from theperimeter of the tank as it goes inwards, in order to obtain the samedisplacement capacity the height of the displacement plates should beincreased as the location of the plate gets closer to the rake shaft.Each displacement plate at the same distance from the shaft is at thesame height. Thus the height of the outer displacement plates is lowerthan that of those nearer the centre. In the inner section of thesettling space, where the shorter auxiliary arms are located between thelong rake arms, the displacement capacity increases as the number ofrake arms doubles, and in this way therefore the height of all thedisplacement plates can be left at the same order of magnitude as theheight of the displacement plates at the tip of the rake arms. Fromhere, the height of the displacement plates can again be graduallyincreased towards the centre. The height of the final displacementplates, those closest to the rake shaft, can be kept basically the same,which causes the underflow displacement capacity of the rake arms in thecentral zone to drop. The height of the displacement plates is kept thesame for a distance that corresponds to 15-30% of the length of thetank-length rake arms. When operating in this way, the outer mass ofunderflow moving towards the centre takes part in moving the underflowin the centre towards the discharge ports. The pressing caused by theoutward-moving underflow compacts the underflow even more and raises thesolids content of the underflow to be removed.

In the way described above the method of the present invention canprevent the “overcapacity” of underflow displacement. When the powertransmission of the rake is equipped with a frequency converter, theunderflow displacement capacity can be specified to be more suitable forevery situation by adjusting the operating speed of the rake. With themethod now developed, the situation can be avoided where theover-displacement of the underflow results in a pile of underflow in thecentre of the clarification tank. If a pile is formed, it may causeblocking of the underflow discharge ports and in addition, prevent theeven distribution into the settling space of the slurry fed from thefeedwell. This in turn results in disturbance of the clarification ofthe overflow, as mentioned previously.

The method and apparatus according to the invention are describedfurther by means of the attached drawings, where

FIG. 1 is a vertical section of the whole clarification apparatus,

FIG. 2 presents a clarification apparatus according to FIG. 1 as seenfrom above, and

FIG. 3 shows a vertical section of a clarification tank feedwellaccording to the invention.

FIG. 1 shows a clarification apparatus 1, inside which is a feedwell 2,situated in the centre of its upper section. Inside the feedwell islocated a slurry mixing element 3 on its shaft 4. A guiding cone 5attached to the lower section of the feedwell is seen only partially.Settled underflow is moved into a cylindrical clarification tank 6 fromthe edges 7 towards an underflow cone 9 situated in the lower section ofthe central zone 8 of the tank by means of the rake 10. A shaft 11, onwhich the rake is hung, travels through the mixing element shaft 4. Thepower transmission of neither shaft is shown in detail. Below the rakearms 12 are the displacement plates 13, and the diagram also shows thatthe height of the displacement plates varies depending on their distancefrom the rake shaft and on the number of rake arms. The rake supportstructure 14 is above the rake. The central zone 8 is ring-like in shapeand extends outwards from the rake shaft a distance of 15-30% the lengthof the long rake arms 12. The height of the displacement plates in thecentral zone can be considered standard. The rake arms are parallel withthe bottom 15 of the clarification tank, in other words sloping downtowards the rake shaft and centre of the tank. The clarification tankthus deepens towards the centre. The clarified overflow is removed fromthe upper section of the tank into its own discharge launder 16.

FIG. 2 presents the clarification apparatus of FIG. 1 as seen fromabove. This shows that the rake 10 is equipped in this case with twolong rake arms 12 and with two shorter auxiliary arms 17 between them.The drawing shows one rake arm with a supporting beam 18 of the armsdrive gear and the others without. The drawing also shows the rakesupporting bars 19. The distance of the displacement plates 13 from eachother is the same all along the rake arm. As shown in the drawing, thedisplacement plates are at an angle of about 30 degrees to the rake arm(in relation to the radius). The length of the displacement plates isthe same all along the rake arm. The length (width) of the displacementplates is determined so that the tail end of the previous plate and thefront end of the next plate overlap by 10-20%. The functional efficiencyof the displacement plates is also improved by the fact that the platesare at a 5-25 degree angle to the vertical plane i.e. they are slightlyinclined towards the rake shaft. As the rake rotates the underflow isdisplaced towards the centre by the ploughing effect of the displacementplates. In addition, as a result of the positioning of the displacementplates described above, the underflow also rises a little over thedisplacement plates. Thanks to this simple procedure the displacementplates do not compact the thin layer of underflow below the rake, but onthe contrary, keep it loose thus preventing the rake from getting wedgedstuck.

FIG. 3 presents in more detail the clarification tank feedwell 2, whichis upright cylindric in shape. The slurry is fed into the well via apipe 20, and the flocculant via another pipe 21. Both the slurry and theflocculant can be fed in a gentle stream into the feedwell, because theyare mixed together inside the well by means of a mixing element 3. Thedrawing shows that the helix mixer shaft 4 is built as a socket so thatboth the mixing tank rake shaft 11 and also the coupling flange 22 ofthe rake shaft fit through it. The mixing element 3 is comprised of twotubes 23, which rotate around the shaft at ⅓-2 revolutions. The tubes 23are supported on the shaft by means of support elements 24. The mixingdirection 25 is rising. The diameter of the mixing element is 40-80% ofthe diameter of the feedwell.

The feedwell 2 is also equipped with baffles 26, which are locatedevenly on the outer edge 27 of the feedwell. Both the slurry and theflocculant are preferably guided into the feedwell nearby the baffles onthe front side of the mixing. Some of the baffles are essentially thesame height as the feedwell, but some, preferably every second one, areonly ⅓ of the height of the feedwell. All the baffles are supported torise upwards from the bottom of the feedwell 28. The baffles number from12-24. Slurry discharge ports 29 are located in the bottom of thefeedwell always between the baffles so that they are in front of thebaffles in regard to the rotation direction of the mixing element.According to the feedwell of this invention, the flocculant is mixedinto the slurry quickly, but when the residence time of the slurry inthe feedwell is regulated between 3-15 minutes, the slurry isflocculated well before it is guided onwards.

Attached to the bottom of the feedwell is the guiding cone 5, whichopens downward, and the feedwell discharge ports open into the inside ofthe guiding cone. The guiding cone is comprised of one or severalconical surfaces and is preferably, although not necessarily, openinside. FIG. 3 shows two conical surfaces, of which the upper one 30opens downwards at a 20-45 degree angle (from the horizontal). The upperconical surface is tightly fixed to the lower conical surface 31, whichopens downwards at a 45-75 degree angle (from the horizontal). Locatedin the lower section of the guiding cone are the slurry guide ports 32.The ports number for example between 16-32 and the outflow of slurryfrom them can be adjusted within the range of 0.05-0.2 m/s. The loweredge 33 of the guiding cone is whole and preferably vertical. Thepurpose of the guiding cone is to prevent the slurry being fed into theclarification tank from flowing strongly towards the surface of theunderflow layer, which would jeopardise the clarity of the overflowsolution. The guiding cone can be dimensioned so that the diameter ofits lower edge is 1.3-2 times larger than the diameter of the feedwell.When the guiding cone is formed of several conical surfaces, thesteepest conical surface preferably accounts for 55-70% of the totalheight of the whole guiding cone.

The invention is described further by means of the following examples.

EXAMPLE 1

It was desired to obtain an even displacement of underflow in anindustrial scale thickener developed from the perimeter of thecylindrical tank towards the underflow well in the centre. The diameterof the thickener was 27 m and the amount of underflow to be moved to thewell was 40 m³/h. An operating speed of 0.075 rpm was chosen for therake system. The attached Table 1 shows how the height of thedisplacement plates increases first from the outer perimeter up to theauxiliary rake arms, where the height of the plates decreases, becausetheir number is doubled. After the minimum point at the tip of theauxiliary rake arms, the height of the displacement plates againincreases towards the centre. The last displacement plates, thoseclosest to the centre are the same height as each other, so the outwardraking pressure compacts the underflow even more.

TABLE 1 Numbering of displacement plates from edge of tank towardsHeight of Displacement centre displacement capacity of plates on Mainrake Auxiliary plates same circumference arms rake arms mm m³/h 1, 2 12043.6 2, 3 125 43.6 3, 4 130 43.8 4, 5 135 43.7 5, 6 140 43.3 6, 7 14843.8 7, 8 156 44.1 8, 9 164 44.2  9, 10 172 44.0 10, 11 180 43.7 11, 12190 43.7 12, 13 200 43.2 13, 14 210 42.6 14, 15 220 41.7 15, 16 1, 2 13045.8 16, 17 2, 3 145 47.2 17, 18 3, 4 160 47.8 18, 19 4, 5 175 47.6 19,20 5, 6 190 46.6 20, 21 6, 7 205 44.8 21, 22 7, 8 220 42.2 22, 23 8, 9220 36.3 23, 24  9, 10 220 30.5 24, 25 10, 11 220 24.5 25, 26 11, 12 22018.7 26, 27 12, 13 220 17.4

EXAMPLE 2

A settling test showed that the slurry is distributed evenly in athickener according to the invention. The slurry used in the tests wasan industrial-scale thickener underflow, which had been obtained byneutralising the waters containing iron(III), chrome(III) and nickel(II)and sulphate generated in refined steel pickling. 52% of the solids inthe slurry were gypsum and the rest metal hydroxides. The diameter ofthe test thickener was 1100 mm and the effective depth of the cylindersection 340 mm. The conical bottom inclined at an angle of 9.5° towardsthe centre. The rake was in principle according to that described inexample 1. The diameter of the feedwell of the thickener was 172 mm andthe effective depth 315 mm. The baffles and guiding cone were asdescribed in the preamble. The feedwell mixer was a helix-type i.e.including two tubes around the shaft rising one revolution in the mixingdirection, and at a constant distance from the shaft. The diameter ofthe mixer was 110 mm and the depth 252 mm.

The underflow previously settled in the settling tests was pumped backinto circulation via the thickener feedwell. Water was also fed into thefeedwell in the proportion of three parts to one part underflow. Waterseparated in the thickener was removed as thickener overflow. The feedof underflow and water was increased in proportion and then lowered inthe same ratio to determine the separation capacity and separationefficiency of the thickener. At first a flocculant was not used, so thatthe performance characteristic of the thickener was largely dependent onhow evenly the slurry flowed out of the feedwell when it was firstmixed. In the test, the speed of revolution of the mixer was 127 rpm andthat of the rake 0.4 rpm. In the final stages of the test the clearseparated water layer developed as follows:

TABLE 2 Time Water feed Underflow feed Water layer min l/h l/h Mm  00142 45 119   15 144 45 94 114 131 36 87 170 131 35 75 195 101 35 82

According to the results described above, the maximum feed of the testthickener can be determined as about 120 l/h water and 40 l/h underflow.The input flows can be raised surprisingly high, when taking intoaccount the settling properties and the fact that no flocculant was usedin the test. The water layer that separated out was completely clear,which also indicates that the mixer used in the feedwell evens out thethickener feed. The conventional thickener, from whence the underflowused in the test came, was not able to achieve the same performance,since the separated overflow remained cloudy.

EXAMPLE 3

The test in example 2 was carried out using the flocculant FennopolA305, which was dosed as a 0.5 g/l-solution of 136 mg/kg solids. Theoverflow layer was in the order of 100 mm and completely clear, when thefeed was 360 l/h water and 120 l/h underflow. The feedwell mixer wasthus able to mix the water, underflow and flocculant homogeneously anddistribute it evenly into the slurry layer of the thickener.

1. A method for clarifying and/or thickening a solution containingsolids, comprising feeding a slurry via a feedwell into a settling spacefor clarification, removing the settled solids or underflow via adischarge port in the center of the settling space and removing theclarified solution or overflow as an overflow, and displacing theunderflow from the edges of the space towards the center via a rake inthe settling space rotating on its shaft with displacement elements onsaid rake radially to the tank, moving the underflow via displacementelement displacement plates at the same distance from the shaft as theyrotate from a ring-shaped cross-section towards the center, thedisplacement capacity of the underflow displacement elements onconcentric ring-like cross-sections being basically the same from theedges of the settling space to the central area and the displacementcapacity on the innermost concentric ring-like cross-sections of thecentral area being smaller than on the other cross-sections.
 2. A methodaccording to claim 1, wherein the underflow displacement occurs usingdisplacement plates attached to the displacement elements, forming anangle of about 30 degrees in relation to the displacement elements andtank radius.
 3. A method according to claim 1, wherein the central areaextending from the rake shaft a distance of 15-30% of the length of thesettling space-length rake, has a smaller displacement capacity than theouter ring-like cross-sections.
 4. An apparatus for clarifying and/orthickening a solids-containing solution, whereby the apparatus comprisesa clarification and/or settling tank, which is equipped with a feedwellfor the slurry to be settled, discharge ports for the settled solids orunderflow in the centre of the settling space, a discharge launder forthe clarified solution or overflow, and a rake rotating on its shaft,which moves the underflow from the edges of the tank to the center,where said rake has at least two arms reaching the edge of the tank in aradial direction, in order to keep the underflow displacement capacityconstant from the edges of the clarification and/or thickening tank tothe centre, there are displacement plates attached to the rake arms,with a height dependent on the distance of the displacement plate fromthe rake shaft and the number of rake arms, and the height of thedisplacement plates in the central area, which is ring-like in shape andreaches from the rake shaft outwards for a distance of 15-30% of thelength of the long rake arms, is kept basically the same.
 5. Anapparatus according to claim 4, wherein the height of the rake armdisplacement plates increases from the edge of the tank towards the rakeshaft as far as the central area.
 6. An apparatus according to claim 4,wherein the rake is equipped with at least two auxiliary arms, thelength of which is about half the length of the arms reaching the edgeof the tank.
 7. An apparatus according to claim 6, wherein the height ofthe rake arm displacement plates increases from the edge of the tanktowards the rake shaft as far as the tip of the auxiliary arm.
 8. Anapparatus according to claim 6, wherein the height of all the rake armdisplacement plates increases from the tip of the auxiliary arms as faras the central area.
 9. An apparatus according to claim 4, wherein thedisplacement plates are attached to the rake arms at an angle of about30 degrees in relation to the longitudinal direction of the arm and thetank radius.
 10. An apparatus according to claim 4, wherein thedisplacement plates are inclined forwards at an angle of 5‥25 degrees inrelation to the vertical axis.
 11. An apparatus according to claim 4,wherein each of the displacement plates at the same distance from therake shaft are the same height.